This invention relates to a sensor, such as pressure and flow sensors, actuators accelerometers, etc and particularly to features of such devices to enable testing of their functionality.
In recent times there have been many advances in the development of micromachined devices for measuring different physical properties such as changes in acceleration, pressure and flow. Such devices are useful in many applications and are used widely in the aerospace and automotive industries, for example. Such devices use flexible parts such as mass spring systems in accelerometers, plates or diaphragms in pressure sensors or some kind of flexible spring elements in the more general case. For example, one approach to acceleration sensing in such application is to provide a device formed from a semiconductor material, the device having a mass formed from the semiconductor material and suspended with respect to the rest of the device. Changes in acceleration of the device can be detected by measuring the deformation of the device by movement of the mass piezoresistively, capacitively, electrostatically or thermally. Whilst such approaches are effective, problems can occur if the device becomes damages or in-effective in use. This is because it is difficult to distinguish between signals arising from external acceleration, from damage occurring external to the device, or lack of signal due to damage in-device whilst it is operating.
Another approach used in accelerometers is to excite a device at a resonant frequency. With such devices there is a detector on the device which allows for detection of the resonant frequency of the device, which is dependent upon external accelerative forces. Thus by investigating the resonant frequency one can find the external acceleration. Indirectly this can provide a continuous functionality-test, due to the continuous detection of the resonant frequency, but is costly to achieve because of the requirements for additional circuitry, etc.
The present invention seeks to provide a more cost effective device with both combination of functionality and the ability to provide test indicators (or self testing).
The present invention seeks to provide a micro machined device which is reliable and cost effective, but which can be tested regularly, or continuously, and whilst in use, to ensure proper functionality.
According to the present invention there is provided a sensor formed from a semiconductor material, the device comprising:
a support frame;
a sensing element;
means for vibrating the sensing element at a frequency corresponding generally to a first resonant frequency vibration mode;
error detection means for detecting the resonant frequency vibration mode, the output of the error detection means being indicative of the existence or otherwise of an expected response of the resonant frequency vibration mode to the excitation; and
means for detecting the deformation of the sensing element to provide an output indicative of the parameter to be sensed, the deformation detecting means and error detection means being formed from the same elements, wherein the means or detecting the deformation are the same as the means for setting up vibrations and can electronically detect both the error detection signal as well as the sensing signal.
The sensing element may be a membrane, and the sensor may be a pressure sensor. The sensing element may be a mass or masses and support beam or beams, and the sensor may be an accelerometer.
The sensing element may be a spring. The vibrating means may be one or more resistive elements. The resonant frequency detection means may be one or more piezoresistors positioned on the device. The support and sensing element may be formed from a single semiconductor substrate. The semiconductor may be silicon.
Using piezoresistors/capacitors etc. deformation in the springs/mass, plate/diaphragm or some kind of spring elements due to external forces will be detected. The operating frequency band will be below the fundamental resonant (first) mode leading to simple separation of segments. Furthermore, as excitation at one of the resonant frequencies is performed, this resonant frequency will be detectable using piezoresistors, capacitive elements etc. The response generated from the device may thus exhibit two pieces of important information, namely information on functionality, that is, information on whether it is working properly, and external acceleration, pressure, flow, etc. The self test of the device of the invention has the advantage that it can be performed during use of the accelerometer, either continuously or at regular intervals to reduce power consumption.